		<title> Flow Control </title>

		<h1 align="center">Flow Control</h1>



<!--INDEX "Flow control" dlproto dlecho Xon-Xoff Etx-Ack -->



A flow-control protocol is selected to ensure that the host does not

send data too fast for the target to receive. The need for this can

arise because, although the target system may be able to read a record

at 9600 baud, the target system may need time to process that record

before it can read the next record.<p>



The environment variables dlproto and dlecho are used specify the

flow-control protocol.<p>



The following table summarizes the four flow-control protocols supported by

this <a href="mondef.htm">Monitor</a>.<p>



<table border cellpadding="5" width="95%">
  <tr bgcolor="#CCCCFF"> 
    <th>Terminal Emulator Sends</th>
    <th>Target Returns</th>
    <th>Set</th>
    <th>Application</th>

</tr>



  <tr bgcolor="#CCCCCC"> 
    <td valign="top"> Line terminated by carriage return</td>

    <td valign="top">Echoes same line</td>

    <td valign="top" nowrap><tt> dlecho = on<br>

dlproto = none<tt></tt></tt></td>

    <td valign="top"> Terminal Emulator waits for full line to be echoed before 
      sending next line. </td>

</tr>



<tr>

<td valign="top"> Line terminated by carriage return</td>

<td valign="top">Returns line-feed character</td>

<td valign="top" nowrap><tt>

dlecho = lfeed<br>

dlproto = none<tt></tt></tt></td>

<td valign="top">

Terminal Emulator waits for line-feed to be echoed before sending next line.

</td>

</tr>



  <tr bgcolor="#CCCCCC"> 
    <td valign="top"> Line terminated by carriage return</td>

    <td valign="top">Returns Xoff and Xon characters</td>

    <td valign="top" nowrap><tt> dlecho = off<br>

dlproto = XonXoff<tt></tt></tt></td>

    <td valign="top"> Terminal Emulator sends until Xoff. Resumes sending after 
      Xon. </td>

</tr>



<tr>

<td valign="top"> Line terminated by ETX character</td>

<td valign="top">Returns Ack character</td>

<td valign="top" nowrap><tt>

dlecho = off<br>

dlproto = EtxAck<tt></tt></tt></td>

<td valign="top">

Terminal Emulator sends line terminated by Etx. Sends next line after Ack

(see note).

</td>

</tr>



</table>
<p> <b>Note:</b> The Xon-Xoff protocol is often not as reliable a method of flow-control 
  as it might first appear. This is because many host systems do not shut off 
  the flow of characters immediately upon receiving an Xoff. They may continue 
  to transmit 20 or 30 bytes before finally stopping. Unfortunately these characters 
  will not be seen by the monitor because it is not interrupt driven, and will 
  be busy processing the previous line.
<h2>See Also:</h2>
<blockquote>
  <p><a href="c_stty.htm">stty</a>, <a href="htermhlp.htm">hyper term help</a>, 
    <a href="terms.htm">terminals</a> and <a href="tiphelp.htm">Unix tip help</a></p>
</blockquote>
<p>



<p><hr>

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